1. Field of the Invention
The present invention relates to an apparatus and method for attaching a semiconductor die to a lead frame or other type of connector. More particularly, the present invention relates to relocating electric contact points of a semiconductor die to the periphery of the semiconductor die through a plurality of conductive traces. The leads of the lead frame extend over the conductive traces proximate the semiconductor periphery and directly attach to and make electrical contact with the conductive traces in a variety of arrangements or configurations. Alternately, a connector may be used to contact a portion of the end of a conductive trace located at the periphery of a semiconductor die.
2. State of the Art
Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are goals of the computer industry. Greater integrated circuit density is primarily limited by the space or xe2x80x9creal estatexe2x80x9d available for mounting a semiconductor die on a substrate such as a printed circuit board. Conventional lead frame design inherently limits package density for a given semiconductor die size because the die-attach paddle of the lead frame must be larger than the die to which it is bonded. The larger the semiconductor die, the less space that remains around the periphery of the die-bonding pad for wire bonding. Furthermore, the wire bonding pads on the standard lead frame provide anchorage for the leads when the leads and the semiconductor die are encapsulated in plastic. Therefore, as the die size is increased in relation to a given package size, there is a corresponding reduction in the space along the sides of the package for the encapsulating plastic which joins the top and bottom of the plastic body at the mold part line and anchors the leads. Thus, as the leads and encapsulant are subjected to the normal stresses of subsequent forming and assembly operations, the encapsulating plastic may crack, compromising package integrity and substantially increasing the probability of premature device failure.
Also, since lead frames are designed for use with a semiconductor die having a specific pattern of bond pads located on the active surface thereof, it is desirable to have the flexibility of changing the bond pad locations of a die so that an existing lead frame design may be used with differing types of die material.
For example, one method of chip attachment which reduces the die size is a so-called xe2x80x9clead-over-chipxe2x80x9d (xe2x80x9cLOCxe2x80x9d) arrangement. Conventional LOC devices have a plurality of leads which is disposed on and attached to an active surface of a semiconductor die, thus the name lead-over-chip. A primary advantage of LOC is that the ratio between the size of the semiconductor die and the size of a package which encapsulates the semiconductor die is high. This advantage is achieved because the die attach paddle is not required since the semiconductor die is instead attached to the leads.
U.S. Pat. No. 4,862,245 issued Aug. 29, 1989 to Pashby et al. (xe2x80x9cthe ""245 patentxe2x80x9d) illustrates an LOC arrangement on a semiconductor die (see FIG. 10). The leads 16 are extended over a semiconductor die 10 toward a central or axial line of bond pads 14 wherein bond wires 12 make the electrical connection between the inner ends of leads 16 and the bond pads 14. In wirebonding, the bond wires 12 are attached, one at a time, to each bond pad 14 on the semiconductor die 10 and extend to a corresponding lead or trace end 16 on a lead frame or printed circuit board (not shown). The bond wires 12 are generally attached through one of three industry-standard wirebonding techniques: ultrasonic bondingxe2x80x94using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bondingxe2x80x94using a combination of pressure and elevated temperature to form a weld; and thermosonic bondingxe2x80x94using a combination of pressure, elevated temperature, and ultrasonic vibration bursts. Film-type alpha barriers 18 are provided between the semiconductor die 10 and the leads 16, and are adhered to both, thus eliminating the need for a separate die paddle or other die support aside from the leads 16 themselves. The configuration of the ""245 patent assists in limiting the ingress of corrosive environmental contaminants to the active surface of the die, achieves a larger portion of the circuit path length encapsulated in the packaging material applied after wire bonding, and reduces electrical resistance caused by the bond wires 12 by placing the lead ends in closer proximity to the bond pads 14 (i.e. the longer the bond wire, the higher the resistance). Although this configuration offers certain advantages, it requires that bond wires 12 be individually attached between the bond pads 14 and the leads 16. Bond wires have an inherent problem called bond wire sweep. When encapsulating a bare die assembly, the die assembly is generally placed in a mold with a molten encasing material being injected into the mold whereby the encasing material surrounds the die assembly and the material conforms to the mold. However, this process causes stresses on the bond wires. Since the molten encasing material is viscous, it tends to place directional forces on the bond wires as the encasing material is injected into the mold. These directional forces cause the bond wires to stretch, which can, in turn, cause the bond wires to short with adjacent bond wires or bond pads or be pulled from a bond pad or lead to which the wires are bonded.
U.S. Pat. No. 5,252,853 issued Oct. 12, 1993 to Michii illustrates an LOC arrangement on the semiconductor die which does not use bond wires (see FIG. 11). The leads 22 are extended over a semiconductor die 20 toward centrally located bond pads 24 (shown in shadow). The leads 22 extend to a position over their respective bond pads 24 wherein the leads 22 are bonded directly to their bond pads 24 with TAB attachment. Although this direct bonding of the lead to the bond pad eliminates the need for wirebonding, it still requires lengthy leads to make electrical contact between the bonds pads and the lead frame. Film-type alpha barriers 26 are also provided between the semiconductor die 20 and the leads 22.
Therefore, it would be advantageous to develop a technique and a device for increasing integrated circuit density by reducing lead width and reducing bond pad size, using non-complex lead frame configurations, and eliminating bond wires, while using commercially-available, widely-practiced semiconductor device fabrication techniques.
The present invention relates to an apparatus and method for attaching a semiconductor die to a lead frame or other type of connector, such as a clip type. Electric contact points of the semiconductor die of the present invention are relocated to the periphery of a semiconductor die and are in electrical contact with a lead frame or connector. The semiconductor die may be in electrical contact with a lead frame through at least one lead which extends over and directly attaches to its respective electric contact point on the semiconductor die periphery, or through one lead which extends over and is attached to a die contact point with electrical contact being made to the electrical contact point of the die by means of a wire bond, or through one lead which extends adjacent the edge of a die with electrical contact being made to the electrical contact point of the die by means of a wire bond.
The apparatus is constructed by first forming a semiconductor die on a semiconductor wafer. A plurality of electric contact points, such as bond pads, is disposed on an active surface of the semiconductor die. A plurality of conductive traces is formed on the semiconductor die active surface to make a conductive route between each electric contact point and a position proximate to the semiconductor die periphery. A plurality of edge electric contact points may be formed on the periphery of the semiconductor die active surface during the formation of the conductive traces.
The conductive traces can be formed by a number of industry standard techniques, such as: depositing a conductive material on the active surface of the semiconductor die, patterning, and etching the conductive material; depositing a conductive paste on the semiconductor die active surface by silk screening the conductive traces directly thereon; directly extruding a conductive paste to form the conductive traces, or applying an insulative material on the semiconductor die active surface, etching a trough in the insulative material, filling the trough with a conductive material, and removing the excess material. These methods are less expensive than relocating the electric contact points during the semiconductor die fabrication process.
Although the formation of the conductive traces is preferably carried out on the semiconductor wafer, it is understood that the traces can be formed on each semiconductor die after the semiconductor dice have been cut from the semiconductor wafer.
After the electrical traces have been formed on the semiconductor die and the semiconductor die has been cut from the semiconductor wafer, a lead frame is attached to the semiconductor die. In one instance, a plurality of leads from the lead frame is directly attached directly to and forms an electrical contact with the edge electric contact points of the semiconductor die. The direct attachment of the leads eliminates the need for bond wires, which reduces the cost of the apparatus. In another instance, a plurality of leads from the lead frame is directly attached to the die with electrical contact being made to the contact points of the semiconductor die by means of wire bonds. In yet another instance, a plurality of leads from the lead frame is terminated adjacent an edge of the semiconductor die with electrical contact being made with contact points of the semiconductor die by means of connectors.
In one instance, since the present invention provides neither a die-attach paddle nor a plurality of lengthy leads to provide support for both the semiconductor die and attached lead frame, the semiconductor device fabrication technique for the semiconductor die of the present invention may have to be slightly modified over present semiconductor device fabrication techniques to insure that no stresses on the lead frame attached semiconductor die occur prior to the encapsulation step. Such a fabrication technique modification may include providing clips on the lead frame to hold the semiconductor die. Although modifying the fabrication process is a disadvantage, the disadvantage is far outweighed by the benefits realized by the present invention. Since the leads are not required to provide support, they can be designed to be narrower in width. The narrower lead width allows the edge electric contact points to be smaller than relocated electrical contact points. The smaller edge electric contact points allow the semiconductor die size to be reduced or allow a greater number of edge electric contact points to be placed on the semiconductor die periphery. The narrower lead width also results in a smaller lead pitch which serves to reduce the cost of the apparatus. Furthermore, attachment of the leads at the semiconductor die periphery eliminates the need for a film-type alpha barrier between the semiconductor die and the lead, which further reduces the semiconductor die cost.